Low temperature curing polyimides

ABSTRACT

A low temperature curing, linear polyimide is produced by substituting diaminostilbene not in excess of 50 mole percent for the primary diamine in the polyimide backbone chain, and upon curing in the presence of a free radical producing catalyst, and optionally, an ethylenically terminated cross-linking agent, a high performance, thermosetting polyimide is formed.

United States Patent Jones et al.

[ Dec. 16, 1975 LOW TEMPERATURE CURING POLYIMIDES Inventors: Robert J. Jones, Hermosa Beach;

Howard N. Cassey, Long Beach, both of Calif.

Assignee: TRW lnc., Redondo Beach, Calif.

Filed: Feb. 15, 1974 Appl. No.: 443,058

US. Cl. 260/47 CP; 260/47 CB; 260/474 A; 260/49; 260/65; 260/77.5 R; 260/78 TF; 260/78 UA Int. Cl. C08G 73/10 Field of Search. 260/47 CP, 47 CB, 65, 78 TF, 260/77.5 R, 78 UA, 474 A References Cited UNITED STATES PATENTS 4/1965 Edwards 260/78 l/l97l 4/197] Rao et al. 260/453 Bargain 260/47 Primary ExaminerLester L. Lee Attorney, Agent, or FirmDaniel T. Anderson; Alan D. Akers; Willie Krawitz 8 Claims, No Drawings LOW TEMPERATURE CURING POLYIMIDES The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of Defense.

BACKGROUND OF THE INVENTION Polyimides are attractive for use where high performance polymers are required. Generally polyimides can withstand melting when exposed to temperatures of 500C for extended periods of time. While many uses have been found for polyimides, their uses have been limited in many areas by their low elongation and by their high processing temperatures. For example,

US. Pat. No. 3 l79,634 teaches a polyimide which has as follows:

0 II C an elongation of 22% which is imidized at a temperature of 325C, and US. Pat. No. 3,533,997 teaches a SUMMARY OF THE INVENTION Low temperature curing, linear polyimides are produced by reacting a primary diamine or a diisocyanate with a dianhydride or a tetracarboxylic acid followed by curing of the resulting polyimide in the presence of a free radical producing catalyst. The primary diamine must be diaminostilbene not in excess of 50 mole percent. Optionally, a cross-linking agent in the amount of up to equal molar amounts with the diaminostilbene may be included. The cross-linking agent must have an ethylenically unsaturated structure in the terminal position on either end of the compound.

Ideally, the polyimide structure may be represented wherein R is a tetrafunctional radical selected from the group consisting of cw we and an alkdilycliene ranging from C to C wherein R is a difunctional radical selected from the group consisting of SO O, S-, CO, and

R is a difunctional radical selected from the group consisting of CH2 l (CH2CH2O) -(cH cH cH o) v, CH2 y y 30 wherein R is a difunctional radical selected from the and group consisting of O, S, SO CO. CH;. -C H.,. and C H R is a difunctional radical selected from the group consisting of R, R.-.

wherein R is selected from the group consisting of wherein R is a monofunctional radical selected from the group consisting of hydrogen and fluorine; R* is a radical from the peroxide; and R is a difunctional radical selected from the group consisting of bis( 3 ,4-dicarboxyphenyl)ether dianhydride ethylene tetracarboxylic dianhydride naphthalene- 1 ,2,4,5-tetracarboxylic dianhydride naphthalenel ,4,5,8-tetracarboxylic dianhydride y is an integer from 1 to 150; Z is a fractional amount 2 from O to and including t; t is a fractional amount up to one half; and x is an integer from 8 to 200.

The procedure for making the polyimide comprises reacting the dianhydride or tetraacid, the diaminostilbene and the primary diamine to produce a polyimideacid. If a cross-linking agent is used, it may be blended into the initial mix or subsequently included when the free radical catalyst is added. After the free radical catalyst has been added the mixture is heated to imidize the polyimide-acid and simultaneously or subsequently activate the free-radical producing catalyst and the cross-linking agent to produce the cured cross-linked polyimide. Polyimides according to this invention may be used as elastomers, coating, films, adhesives, and composites.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Low temperature curing polyimides are produced by reacting a tetracarboxylic acid or a dianhydride with diaminostilbene and a primary diamine or diisocyanate and curing the mixture in the presence of a free radical producing catalyst. The diaminostilbene may be present in any amount but not in excess of 50 mole percent of the total amount of primary diamine present, an optionally, an ethylenically terminated cross-linking agent may be added in amount equimolar with the diaminostilbene present.

Examples of the dianhydrides which have been found to be suitable in the practice of this invention, may be selected from the following list which is representative of a few of the dianhydrides:

pyromellitic dianhydride benzophenone tetracarboxylic dianhydride 2,3,6,7-naphthalene tetracarboxylic dianhydride 3,3',4,4'-diphenyl tetracarboxylic dianhydride l,2,5,6-naphthalene tetracarboxylic dianhydride 2,2,3,3 '-diphenyl tetracarboxylic dianhydride 2,2-bis( 3,4-dicarboxyphenyl)propane dianhydride 3,4,9, 1 O-perylene tetracarboxylic dianhydride we ea LII decahydronaphthalene- 1 ,4,5,8-tetracarboxylic dianhydride 4,8-dimethyl- 1 ,2,3,5 ,6,7,-hexahydronaphthalene- 1,2,5 ,6-tetracarboxylic dianhydride 2,6-dic hloronaphthalenel ,4,5 ,S-tetracarboxylic dianhydride 2,7-dichloronaphthalene-l ,4,5,8-tetracarboxylic dianhydride 2,3 ,6,7 -tetrachloronaphthalenel ,4 ,5 S-tetracarboxylic dianhydride phenanthrenel ,8,9, 1 O-tetracarboxylic dianhydride cyclope ntane-l ,2,3 ,4-tetracarboxylic dianhydride pyrrolidine-2,3,4,5-tetracarboxylic dianhydride pyrazine-2,3,5,6-tetracarboxylic dianhydride 2,2-bis( 2,3-dicarboxyphenyl)propane dianhydride l, 1 -bis( 2,3-dicarboxyphenyl )ethane dianhydride l 1 -bis( 3,4-dicarboxyphenyl )ethane dianhydride bis( 2,3 -dicarboxyphe nyl )methane dianhydride bis( 3 ,4 -dicarboxyphe nyl )methane dianhydride bis( 3,4-dicarboxyphenyl)sulfone dianhydride benzenel ,2,3,4-tetracarboxylic dianhydride l,2,3,4-butane tetracarboxylic dianhydride thiophene-2,3,4,5-tetracarboxylic dianhydride bis( 3 ,4 -dicarboxyphenoxyphenyl )sulfone dianhydride It is to be understood that the corresponding tetraacids of the above-listed dianhydrides are equally suitable.

Alternatively, the tetraacid compounds may be illustrated by the following structural formula:

wherein R is a tetrafunctional radical selected from the group consisting of All of the polyimide formulations in this invention require the presence of some diaminostilbene. Diaminostilbene provides the cross-linking site for the polyimide backbone chain, and should be present in amounts not over 50 mole percent of the total amount of the primary diamines present.

Primary diamines used in this invention are present in an equal molar amount with the dianhydride. As stated above, not over 50 mole percent, and preferably to mole percent, of the diamine should be the particular diamine, diaminostilbene. The remaining primary diamines may be selected from any of the following:

para-phenylene diamine 3-methylheptamethylene diamine 4,4-dimethylheptamethylene diamine 2, l l-diamino-dodecane l,2-bis-( 3-amino-propoxy)ethane benzidine 2,2-dimethyl propylene diamine 3-methoxy-hexamethylene diamine 3,3 '-dimethyl benzidine 2,S-dimethylhexamethylene diamine 2,5-dimethylheptamethylene diamine S-methyl-nonamethylene diamine 2, l 7-diamino-eic0sadecane 1,4-diamino-cycl0hexane l, lO-diaminol l O-dimethyl decane l 1 Z-diamino-octadecane structurally, the primary diamines may be illustrated group consisting of and wherein R is a difunctional radical selected from the group consisting of -O, S-, SO CO, CH C H and C H and y is an integer from 1 to 150.

Alternatively, diisocyanates may be used in place of or in conjunction with the primary diamines. When diisocyanates are used, not over mole percent, and preferably 10%-2O mole percent, of the diaminostilbene is substituted for the diisocyanate. Diisocyanates suitable for use in the invention have the structural formula:

O=C=NR,,-N=C=O wherein R is a difunctional radical selected from the group consisting of l-isopropyl-2,4-metaphenylene diamine m-xylylene diamine hexamethylene diamine heptamethylene diamine octamethylene diamine nonamethylene diamine decamethylene diamine diamino-propyl tetramethylene diamine CF CF CE; C CF21 For further modification of the polyimides, a crosslinking agent can be included. Cross-linking agents may be selected from a wide variety of materials, however, all of the cross-linking agents must have an ethylenically unsaturated group at the terminal positions on either end of the molecule. Thus, olefins having unsaturated bonds in the first and last position, bis(male)imides and divinyl substituted aromatics, are suitable for use as cross-linking agents in this invention. A list of specific cross-linking agents which are suitable are:

bis(4-maleimidophenyl) methane bis(4-tetrahydrophthalimidophenyl) methane bis[ 4-( difluoromaleimido )phenyl] methane bis[ 4-( difluoromal'eimido )-2, 3 ,5 ,6-tetrafluorophenyl] difluoromethane 1,3-dimaleimidobenzene l ,4-dimaleimidobenzene perfluorol ,3-butadiene perfluorol ,S-hexadiene perfluorol ,7-octadiene l ,3-divinylbenzene l,4-divinylbenzene perfluoro- 1 ,3-divinylbenzene perfluorol ,4-divinylbenzene l ,2-dimaleimidoethane l ,6-dimaleimidohexane Because the cross-linking agent is believed to react at the ethylenic bond of the diaminostilbene, the amount of the cross-linking agent may vary in any amount from up to an equal molar amount of the diaminostilbene present. Of course, if no cross-linking agent is present or if an amount of cross-linking agent present is less than the amount of the diaminostilbene present, then there will be cross-linking between the diaminostilbene present in the adjacent linear chains. Therefore, by regulating the amount of the diaminostilbene present and the presence and amount of a cross-linking agent, a broad range of polyimide properties can be obtained. For example, a highly aromatic polyimide having a high percentage of diaminostilbene will produce a tough, stiff thermosetting polyimide; whereas, a polyimide having a large number of long alphatic segments and a large number of long chain cross-linking agents will produce a highly elastomeric or compliant polyimide.

Free radical producing catalysts areus'ed to assist in the cross-linking reaction. Peroxides are the preferred 0 0 II II C C II n O Dianhydride Diaminostilbene peroxide catalysts, other catalysts effection reaction of ethylenic unsaturation may be used. The following list of compounds is representative of a few of the free radical catalysts which are suitable for use in this invention. Perfluoroacetyl peroxide, benzoyl peroxide, perbenzoic acid, bisazoisobutyro nitrile, acetyl peroxide, t-butyl peroxide, and 2,5-dimethyl-2,5-bis(t-butyl peroxy) hexane.

Polyimides are prepared by the reaction of an organic dianhydride or tetraacid, diaminostilbene, and a diisocyanate or primary diamine which is subsequently cured in the presence of a free radical catalyst. The free radical catalyst can be added during the initial blending of the components, or it may be added after the mixture is fully imidized. In. order to add the catalyst after the mixture is fully imidized, it is preferable to use a component which will render the imide subject to easy dissolution, e.g. a sulfone, so that the catalyst can be blended in a homogeneous manner throughout the imide.

If the catalyst is added to the components initially, care must be exercised in heating the mixture during imidization if it is desired to avoid cross-linking at that particular time. Of course, the cross-linking reaction and imidization may be carried out simultaneously, however, this is not always desirable. Where separate reactions are desired, a catalyst having a higher activation temperature than the temperature of imidization is required. Thus, the mixture may be heated to a lower temperature range to effect the imidization, and subsequently upon completion of the imidization, the imides may be raised to a higher temperature to activate the catalyst and effect the cross-linking cure. 7

Where a cross-linking agent is employed, it may be added either to the initial blend or subsequently after imidization. Essentially, the same considerations apply to the cross-linking agent as applied to the catalyst, i.e., where the cross-linking agent is added after imidization, the imide must be soluble so as to permit homogeneous blending throughout the imide.

The formation of the polyimide may be illustrated ideally as follows:

Diamine Cross-linking Agent wherein R is a tetrafunctional radical selected from the and an alkdiyldiene ranging from C to C wherein R is group consisting of a difunctional radical selected from the group consist- 4 ing of SO.

@ R 45 R is a difunctional radical selected from the group consisting of wherein R is a difunctional radical selected from the group consisting of O, -S, SO -,CO-, CH C H and C H R is a difunctional radical selected from the group consisting of s 2-! E R.

N- R,. N R*i c c i R5 II II R o o and a a CR1C wherein R is selected from the group consisting of R5 R5 R5 R5 H2 H2 H2 H2 H C H H and 2 2 2 Y is an integer from 1 to 1,50; Z is a fractional amount from 0 to and including t; t is a fractional amount up to one half; and x is an integer from 8 to 200.

As previously discussed, the addition of the catalyst and cross-linking agent maybe madein either of steps I, II, or III, depending upon the type of catalyst used or the type of imide being made.

The invention will be more clearly understood by referring to the examples which follow. These examples are intended to be illustrative only and should not be construed to limit the invention in any way.

EXAMPLE I Exactly 3.867 g (0.0195 mole) of methylenedianiline, 0.630 g (0.0030 mole) of diaminostilbene and 7.860 g (0.0075 mole) of 1000 mol. wt. poly(oxoethylene) diamine were dissolved in 22.5 ml warm dimethyl formamide and placed in a micro blender. Stirring was initiated, then 16.276 gm (0.0300 mole) of predissolved bis(3,4-dicarboxyphenoxyphenyl) sulfone dianhydride in 45 ml dimethyl formamide was slowly added to the micro blender. The reaction mixture was cooled by dry ice during and after the dianhydride addition, and was stirred for an additional five-minute period at fast speed after complete addition of the dianhydride. The resultant polyamic-acid varnish was then placed in an aluminum foil cup, dried and imidized under vacuum at 350F overnight. Exactly 7.70 gm of the polyimide varnish (0.000166 mole diaminostilbene present) at 20% by weight solids in dimethyl formamide was added to 0.030 gm (0.000083 mole) of bis(4,4- diphenylmethane) maleimide and 0.6% by weight of 2,5-dimethyl-2,5-bis( t-butylperoxy )hexane mixed thoroughly, and cast onto an aluminum sheet with a Gardner knife, and dried under vacuum at 350F overnight. The aluminum sheet was dissolved by dilute hydrochloric acid to produce the fully cured cross-linked poly- 15 16 imide. which was insoluble in boiling DMF for more TABLE II than hours- INITIAL SCREENING REgULTS OBTAINED ON FULLY AROMATlC URED POLYMER EXAMPLE II Solubility After Initial Thermoa To exactly 8.67 gm (0.000182 mole diaminostilbene 5 Dimethylac t d niide I-I.,so Apiie rance l l in rii r l present) of the same polyimide varnish was added 0.033 gm 0.000091 mole) of bis(4,4-diphenylme- 355t 600 thane) maleimide and 1.2% by weight of 2,5-dimethyl- Light 2.5-bis(t-butylperoxy)hexane mixed thoroughly, cast gg g onto an aluminum sheet with a Gardner knife, and dried under vacuum at 350F overnight. The aluminum was dissolved by dilute hydrochloric acid to produce the fully cured cross-linked polyimide, which was insoluble in for more than Six hours- "Positive vulcanization was deemed present if sample remained insoluble after a The following table is a summary of the property data l5 twv-hour boil period in dimeihylformamide. and after g our period n H280; at room temperature.

on cured Samples produced Examples I and "Determined on film samples employing ascan rate of 3C/min and 100 ml air flow; above. temperatures reported as first weight loss detected in TGA curve.

TABLE I SUMMARY OF VULCANlZATlON EXPERlMENTATlON AND PROPERTY DATA ON CURED FlLM SAMPLES Quantity of Bl2 Quantity of Solubility Initial" Linear Cross-linking Catalyst in Dimethyl Tensile Properties Thermal Recoverable Gumstock Agents Employed Employed formamide Strength Elongation Stability Elongation Formulation (w/w 71) (w/w 7r) After Cure (Ksi) (75) (F) (7:

Example I 0.38 0.6 No 6900 l 10 615 66 Example ll 0.38 1.2 No 6700 75 620 67 "Quantity of bis(imide) employed corresponds to equal molar amount of unsaturation to that imparted by diaminostilbene.

"Quantity corresponds to an equal molar basis to unsaturation imparted by diam inostilbene.

Positive vulcanization was deemed present if sample remained insoluble in dimethylformamide after a two-hour boil period.

"Average of triplicate breaks deten-nilied on thin films by lnstron analysis employing a crosshead speed of 0.2-in/min.

Determined in film samples employing a scan rate of 3C/min and 100 ml/min N flow; temperatures reported as first weight loss detected in TGA curve. 'Measured on tensile specimens fifteen minutes after elongating to break.

EXAMPLE Hi We claim:

40 1. A process for the preparation of a low temperature To a dry 100 ml. three-necked flask, equipped with a ring polyimide comprising:

mechanical stirrer and a nitrogen inlet, was added a A. blending an organic compound selected from the 1.904 gm (0.0096 mole) methylene dianiline, 0.084 gm group consisting of dianhydride and tetraacid and a (0.0004 mole) diaminostilbene and 19 ml of DMAC, free radical catalyst with diaminostilbene not in Stirring was initiated and the diamine solution was excess of "1016 Percent and the remainder a cooled to 20C by immersing the flask in an ice-water nitrogen compound selected from the group conbath. Bis(3,4-dicarboxyphenoxyphenyl) sulfone diansisting of organic diisocyanate and organic primary hydride 5.425 gm, 0.01 mole) was then added over a diamines whereby a mixture of an amide-acid with IO-minute period to the flask, followed by an additional 50 the catalyst dispersed therethrough is formed;

10 ml of dimethylacetamide. The mixture was stirred B. subsequently heating the mixture in a temperature for two hours and then poured into an aluminum cup range of from 150 to 200C to imidize said amideand imidized under vacuum at 180C for 18 hours. One acid ul y; and

gram (containing 0.0000284 mole of diaminostilbene) C. heating said imide-catalyst in a temperature range of the imidized polymer was placed in a 20 ml roundof from 150 to 250C to activate said catalyst bottom flask and dissolved in 5 gm of dimethylacetamwhereby a compliant polyimide is formed.

ide by stirring at room temperature for 20 minutes. To A process rd ng t laim 1 h r in said atathe solution was added 0.0051 gm (0.0000142 mole) y is added after d am de-aci iS m bis(4,4'-diphenylmethane) maleimide and 0.0165 gm 3. A process according to claim 1 wherein a cross- 2,5 -dimethyl-2,5-bis(t-butylperoxy)hexane. The well linking agent having terminally positioned ethylene mixed solution was cast onto a glass plate with a Gardsubstituents selected from the group consisting of olener knife and cured at 180C for 18 hours. The cured fins, bis(male)-imides, divinylbenzene, and fluoro-subfilm was easily removed from the glass plate by soaking stituted divinylbenzene is blended into the initial mixin warm water. After cure, the polymer was found to be ture in amounts up to equal molar with said diaminosinsoluble in concentrated sulphuric acid at room temtilbene. perature and in boiling dimethylacetamide. 4. A process according to claim 1 wherein said or- The following Table II gives results obtained on the ganic compound has the structural formula selected polymer produced in Example Ill. from the group consisting of o 0 0 wherein R is a difunctional radical selected from the li i! l group consisting of -O, S, SO CO, and -CH C H and C H and is an integer Ho c c-on c C from 1 to 150.

g y l 6. A process according to claim 1 wherein said organic diisocyanate has the structural formula 0=C=N-R,. N=C=o wherein R is a difunctional radical selected from the group consisting of wherein R is a tetrafunctional radical selected from the CFZ 2 f 0 i group consisting of 3 3 and an alkdilydiene ranging from C to C wherein R is and a difunctional radical selected from the group consisting of -O, --S, -SO CO, CH C iFOCFEiFO CFfl Y 4 r 3 6 a and 25 3 3 F1! -oso o- I n o n o I I e 5. A process according to claim 1 wherein said or- 7. A process according to claim 1 wherein said free ganic primary diamine has the structural formula of radical catalyst is an organic peroxide.

"2N Rt NH2 8. A process according to claim 1 wherein said diaminostilbene is present in amounts ranging from 10 to wherein R is a dlfunctlonal radical selected from the mole percent group consisting of H H c CH l I 7 CH CH O y- I CH CH CH O y- H2C \C /CH2 H H H H C C C C d H an H fa 3 H H c c c c H H2 H H 

1. A PROCESS FOR THE PREPARATION OF A LOWER TEMPERATURE CURING POLYIMIDE COMPRISING: A. BLENDING AN ORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF DISNHYDRIDE AND TETRAACID AND A FREE RADICAL CATALYST WITH DIAMINOSTILBENE NOT IN EXCESS OF 50 MOLE PERCENT AND THE REMAINDER A NITROGEN COMPOUND SELECTED FROM THE GROUP CONSISTING OF ORGANIC DIISOCYANATE AND ORGANIC PRIMARY DISMINES WHEREBY A MIXTURE OF AN AMIDEACID WITH THE CATALYST DISPERSED THERETHROUGH IS FORMED; B. SUBSEQUENTLY HEATING THE MIXTURE IN A TEMPERATURE RANGE OF FRM 150* TO 200*C TO IMIDIZE SAID AMIDE-ACID FULLY; AND C. HEATING SAID IMIDE-CATALYST IN A TEMPERATURE RANGE OF FROM 150* TO 250*C TO ACTIVATE SAID CATALYST WHEREBY A COMPLIANT POLYIMIDE IS FORMED.
 2. A process according to claim 1 wherein said catalyst is added after said amide-acid is formed.
 3. A process according to claim 1 wherein a cross-linking agent having terminally positioned ethylene substituents selected from the group consisting of olefins, bis(male)-imides, divinylbenzene, and fluoro-substituted divinylbenzene is blended into the initial mixture in amounts up to equal molar with said diaminostilbene.
 4. A process according to claim 1 wherein said organic compound has the structural formula selected from the group consisting of
 5. A process according to claim 1 wherein said organic primary diamine has the structural formula of H2N - R1 - NH2 wherein R1 is a difunctional radical selected from the group consisting of
 6. A process according to claim 1 wherein said organic diisocyanate has the structural formula O C N-R8-N C O wherein R8 is a difunctional radical selected from the group consisting of
 7. A process according to claim 1 wherein said free radical catalyst is an organic peroxide.
 8. A process according to claim 1 wherein said diaminostilbene is present in amounts ranging from 10 to 20 mole percent. 